Simply a better server.

Lion Server is now part of Mac OS X Lion. It’s easy to set up your Mac as a server and take advantage of the many services Lion Server has to offer. Here are just a few of the new features that make server deployment faster, easier, and more powerful than ever.

Wow, there were a few cool features up to this point, but having server become part of the core OS is phenomenal. It's like Linux, but the printer will work. Actually, that's a low blow--printers don't work in Windows either.